Spark plug boots are only necessary to prevent accidental discharge from the top of the spark plug to other metal objects in the engine compartment. Additionally, the boot aids in keeping contaminants away from the plug/plug wire connection, which could cause the connection to become faulty. Even though the engine will continue to work with broken spark plug boots, replacing the boot(s) at soon as possible will be best in the end. Unless you used a spark plug wire kit to assemble the wires installed on the vehicle, you will not have a spare boot, and auto part stores do not sell them individually.
For more info, see Tips for Proper Spark Plug Maintenance